Gaseous radiation detectors, particularly those employed as position sensitive detectors, have been known for several decades. One early such detector in the form of a so-called MultiWire Proportional Counter (MWPC) is described by G. Charpak, et al. in an article entitled "The Use Of MultiWire Proportional Counters To Select And Localize Charged Particles", Nucl. Instr. and Meth. 62 (1968) 262. The MWPC described in the Charpak paper consisted of a number of thin anode wires assembled in a plane and mounted between two cathode plates. A potential difference was established between the anodes and the cathode plates to thereby produce an electric field sufficient for avalanche multiplication in a gas medium surrounding the wires.
As used herein, the term "avalanche multiplication" (or "avalanche amplification") refers to a succession of ionization collisions in which an electron or ion is accelerated to produce two more ions by collision. That is to say, one or more electrons in a molecule or ion can be knocked free from the molecule or ion by colliding with an electron or ion which has been accelerated in a high electric field gradient in a region where the molecule or ion is located. Additional ions created by such ionization collisions are then accelerated by the electric field to produce more ions, thus resulting in an avalanche condition.
In an article entitled "Position-Sensitive Detector With MicroStrip Anode For Electron Multiplication With Gases", Nucl. Instr. and Meth. A263 (1988) 351, A. Oed combined many of the MWPC concepts with known photolithography techniques to produce a so-called MicroStrip Gas Chamber (MSGC). Such an MSGC 10 is shown in FIG. 1. Referring to FIG. 1, MSGC 10 includes an electrically insulating substrate 12 upon which a series of metallic anodes 14 and cathodes 16 are patterned as is known in the semiconductor processing art. Typically, anode spacing 18 may be on the order of approximately 50-1000 micrometers. A backside conductive layer 24 is provided on the bottom side of substrate 12, the purpose of which will be discussed hereinafter.
A drift electrode 20 is spaced 22 a few millimeters (typically 3-5) above substrate 12. A gaseous medium 26 exists in the region defined by drift electrode 20 and substrate 12, and is typically comprised of a "counting gas", typically comprising a noble gas, such as Argon for example, with a "quench" gas, such as Isobutane.
In the operation of MSGC 10, a small electric field is established between drift electrode 20 and the anodes 14, and an intense electric field is established between the anodes 14 and cathodes 16. An external radiation source (not shown) initially creates charge pairs within the gaseous medium 26, and the small electric field established between drift electrode 20 and anodes 14 typically draws electrons toward the nearest anode 14 and positive charges toward the drift electrode 20. As the electrons drift sufficiently close to an anode 14, the intense electric field established between the anode 14 and cathode 16 causes the electron to undergo avalanche multiplication in a region near the anode. A resulting "cloud" of positive charges then moves toward, and is collected by, cathode 16 and electrode 20. Generally, the voltages on cathode 16 and electrode 20 are set so that only a very small percentage of the charges move toward electrode 20. The positive charges travel just above the insulating substrate 12 and can attach thereto. Backside conductive layer 24 is thus biased positively to prevent charge from accumulating on the electrically insulating substrate 12.
In a later article entitled "The Micro-Gap Chamber", Nucl. Instr. and Meth. A335 (1993) 69, F. Angelini et al. described improvements to the MSGC wherein the anode and cathode are separated by only a few micrometers. Such a device is known as an MGC, and an example of one such MGC 30 is shown in FIG. 2. Referring to FIG. 2, MGC 30 includes an electrically insulating substrate 32 upon which an electrically conductive cathode layer 34 is formed. A series of insulating strips 38 are then formed on cathode 34 at predetermined intervals. Upon each of the insulating strips, an electrically conductive anode strip 36 is formed so that anode strips 36 have a spacing 40 therebetween within the range of approximately 100-200 micrometers. The MGC 30 attempts to minimize charge accumulation on the substrate 32 by minimizing the amount of exposed substrate 32 and insulating strip 38 surface area. As with MSGC 10, MGC 30 includes a drift electrode 42 having a spacing 44 of a few millimeters (typically 3-5) from substrate 32. A gaseous medium is disposed between drift electrode 42 and substrate 32.
The operation of MGC 30 is very similar to that of MSGC 10, except that the electric field established between the anode 36 and cathode 34 is much more intense for comparable anode/cathode potential differences. This phenomenon is largely due to the decreased spacing between anode 36 and cathode 34, defined by the thickness of insulator 38 which is typically in the range of 2-6 micrometers.
Although the MGC 30 has undeniably increased the electric field intensity between the anode 36 and detecting cathode 34 over that of the MSGC 10 for comparable anode/cathode potential differences (Angelini et al. reports an increase in electric field intensity by approximately a factor of 4 over an MSGC 10), both prior art devices suffer from the same inefficient design of the accelerating electric field gradient. In an avalanche ionization event, the geometry of the avalanche region is directly related to the geometry of the electric field gradient. Unfortunately, a problem common to both the MSGC 10 and MGC 30 is that the geometry of the electric field gradient used therein is not designed to permit optimal control over the geometry of the avalanche region. Referring to FIG. 3, for example, a plot of equipotential lines 48 for MGC 30 is shown in relation to cathode 34, anode 36 and insulator 38. As evidenced from FIG. 3, the equipotential lines 48 indicate that electric field lines (normal to the equipotential lines) extending between the anode 36 and cathode 34 exhibit non-parallelism over the entire avalanche region. As such, the specific geometry of the avalanche region is difficult to control.
A further drawback associated with the operation of a MSGC 10 or MGC 30 type detector is a phenomenon known as photon feedback, which is related to certain physical properties of the types of gases used therein. It is generally known that avalanche multiplication can occur in all gases. However, the choice of a particular gas, or gases, for use in a radiation detector of the type described herein is typically driven by various desirable and/or necessary operational parameters such as, for example, low working voltage, high gain operation, good proportionality, high rate capability, long lifetime and fast recovery to name a few. It is also generally known that avalanche multiplication occurs in noble gases at much lower electric fields than in complex molecules. However, during the avalanche process in a noble gas, excited and ionized atoms are formed which can only return to the ground state through a radiative emission. Thus, inherent in the avalanche ionization of a noble gas is the emission of photons.
A large fraction of the emitted light is due to the radioactive decay of the first excited state of the noble gas and, as such, has an energy above the work function of any metal that might comprise the cathode of a radiation detector. Such photons impinging upon the cathode therefore tend to extract photo-electrons therefrom which then initiate a secondary avalanche condition in the presence of the established electric field. Noble ions thus migrate to the cathode where they neutralize by extracting an electron from the cathode. The balance of energy left after extracting the electron is either radiated as a photon, or by secondary emission, i.e. extraction of another electron from the metal surface of the cathode. Photons emitted during electron-ion recombination, as well as photons emitted by the excited atoms, have sufficient energy to eject photo-electrons from the materials of the detector. These photo-electrons tend to propagate the discharge and produce spurious charge counts. To reduce this so called photon feedback effect associated with the use of noble gases, a "quench gas" is typically mixed with the noble gas, which acts to absorb charge from the ionized noble gas. The quench gas is typically a hydrocarbon gas such as isobutane, although various other gases, such as CO.sub.2 or halogens, may also serve as quench gases. The use of such quench gases, however, tends to lead to deposition of undesirable residue on the electrode surfaces. Furthermore, some quench gases, such as the halogens, are highly reactive. Clearly, operation of such radiation detectors without the need for a quench gas would thus be highly desirable.
What is therefore needed is a radiation detector operable to absorb or otherwise divert the emitted photons so that the need for a quench gas can be drastically reduced or eliminated altogether. Further, if such a radiation detector could be designed such that the avalanche is confined to a dielectric boundary, then the geometry of the avalanche region could be optimized for a particular application. Such a device would more efficiently detect radiation as well as exhibit an improved signal-to-noise ratio over prior art devices.